jtice
Flashaholic
How do you measure the forward voltage of an LS ????
I have a digital Fluke meter.
I have a digital Fluke meter.
How to measure the forward voltage of an LS ????
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Shark
Enlightened
Meter on DC volt scale Positive lead on the anode side of the led neg lead on the cathode side of the led turn it on the meter will give you the voltage drop across the led which is the forward voltage VF. Oh yea dont forget to sqint your eyes before turning it on.
jtice
Flashaholic
If i understand you correctly....
It doesnt matter what voltage im putting into the LS?
And basically,, the "left over" voltage comming out the other side of the led is the VF?
How do I tell which side is which on the led... pos= ? neg= ?
It doesnt matter what voltage im putting into the LS?
And basically,, the "left over" voltage comming out the other side of the led is the VF?
How do I tell which side is which on the led... pos= ? neg= ?
jtice
Flashaholic
Oh yea, and how exactly do I use the VF? What does it allow me to then calculate? I undertand I need to know the VF of a led, to tell the V I can crank into it.
From the wisdom osmosed from those here, it's my understanding that forward voltage is the specific voltage, to a specific LED, that when given to that LED, produces the target current draw and the desired luminous flux output...
And by that, you would only be able to measure Vf if you know the target luminous flux and current flow... I suspect, if you supply power to a 1W Luxeon Star, and that LS gets up to 350ma current draw and hits its target luminous flux when your power supply hits, say, 3.3V, then your forward voltage for that LS would be 3.3V...
...My theory...
/ubbthreads/images/graemlins/icon6.gif
And by that, you would only be able to measure Vf if you know the target luminous flux and current flow... I suspect, if you supply power to a 1W Luxeon Star, and that LS gets up to 350ma current draw and hits its target luminous flux when your power supply hits, say, 3.3V, then your forward voltage for that LS would be 3.3V...
...My theory...
/ubbthreads/images/graemlins/icon6.gif
whiskypapa3
Enlightened
The short answer is you drive the LS with a *known current* (traditionally 'full blast', 350 mA), that is you 'light it up' and use your DMM to measure the voltage across it. Don't worry about polarity, if it's lit, it's forward biased. Big V means a voltage, the small f means to measure the voltage when *forward* biased (on), the specs say how much.
If you're driving from a 'stiff source' (as in DD), Vf is drive voltage by definition. Current needs to be determined.
FWIW, I have a pair of 'LM317 fixed current regulators' I use for this, one has 51 ohms for 25 mA testing of 3 and 5 mm units, the other 3.6 ohms for 350 mA testing of LS units.
Under a buck each, always there when I need them, the smaller one has a 9 Volt battery clip.
Doug Owen
If you're driving from a 'stiff source' (as in DD), Vf is drive voltage by definition. Current needs to be determined.
FWIW, I have a pair of 'LM317 fixed current regulators' I use for this, one has 51 ohms for 25 mA testing of 3 and 5 mm units, the other 3.6 ohms for 350 mA testing of LS units.
Under a buck each, always there when I need them, the smaller one has a 9 Volt battery clip.
Doug Owen
